In the fields of audio and music, it is fairly common to apply some type of dynamics processing to audio. Dynamic processing generally focuses on the volume of audio, but not necessarily in a linear fashion. In general, a dynamics processor takes an input audio recording or signal, and produces an output audio recording or signal with the amplitude (volume) adjusted, typically with some time-varying gain.
A common type of dynamics processing is known as compression, wherein the higher the input signal level, the higher the attenuation. Many compressors work using a threshold, in that when the input signal increases in magnitude and crosses the threshold, the attenuation increases by some ratio. Typically, compressors measure signal magnitude by RMS to get an average of the signal energy, although other magnitude measurements, such as peak value, can also be used.
In audio, the term “dynamic range” refers to the range between how loud and how quiet the signal gets. In music, for example, it is common to use dynamics processors to compress the dynamic range to allow for an overall louder signal. Mastering limiters are a special kind of dynamic processor to bring up the overall loudness while preventing clipping. Dynamics processors are also used in other places, for example, to process voice in post-production for broadcast and film.
The most commonly used tool for adjusting the dynamic range of audio is a dynamics processor. They all work roughly the same: there is a level detector, a threshold and ratio that specify how to convert from detected level to target gain, and ballistics for smoothing out the target gain into a smooth gain envelope. For a compressor as already mentioned for example, below the threshold, the target gain is 0 dB (corresponding to no change), and above the threshold, the target gain is reduced by more and more as the threshold is exceeded: i.e. for a ratio of 2:1, for each 1 dB above the threshold that the signal reaches, the target gain will be reduced by 2 dB. Both of these parameters must be tuned depending on the signal. For example, a quieter signal may require a lower threshold. A signal with more dynamic range may require a higher ratio. Different signals may require different types of dynamics processing in combination, i.e. a gate in addition to a compressor. In other words, to achieve the same desired dynamic range, different signals require significantly different types of processing, which must be adjusted by a skilled engineer.
For the smoothing ballistics, an attack time is used when going over the threshold, and a release time is used when going back below it. The attack and release are generally set to allow fast enough response to transients without pumping, while still providing sufficient control over dynamics. If the attack and release times are too long, this may cause pumping, where loudness drops noticeable around a transient. Furthermore, slow ballistics also reduce the effect of the dynamics processing. On the other hand, if they are too short, the gain envelope may vary too quickly, causing noticeable nonlinear distortion.
A disadvantage of typical “threshold and ratio” dynamic processors is that they require a skilled operator to adjust the multiple parameters to obtain high quality results. This often requires trained listening skills as well as exceptional knowledge of the particular dynamic processor. It is difficult for casual users and amateurs to obtain good results with such tools.